Various types of carrier sensing multiple access (CSMA) local area networks are known. These local area networks prevent data transmission loss during overlapping transmissions by sensing or avoiding such overlapping transmissions, i.e. collisions.
It is known to provide secure communications over CSMA/CD networks. Published European Patent Application 0431751A1 describes repeaters for secure local area networks which read the destination address segment of a data packet to determine whether the data packet is destined for a given receiver or class of receivers. Where the data packet is not destined for a given receiver or class of receivers, it corrupts the frame which it is retransmitting, such as by overwriting, so that the frame cannot be read.